Evaluation of Shunt and Series Power ConditioningStrategies for Feeding Sensitive Loads

Bingsen Wang Giri VenkataramananDepartment of Electrical and Computer Engineering

University of Wisconsin - MadisonMadison, WI 53706-1607

Email: bingsen@cae.wisc.edu

Abstract— Proliferation of electronic equipment in commercialand industrial processes has resulted in increasingly sensitiveelectrical loads to be fed from power distribution systems. Inthis paper, series and shunt compensation schemes for feedingsensitive load centers are studied. The purpose of the system-levelstudy is to reveal insights about these two mitigation strategies sothat designers may select an appropriate compensation strategyfor a given application. The paper presents analytical modelsfor comparing series and shunt power conditioning approacheson the basis of power converter rating. Operating strategies,computer simulations and experimental verification of a seriespower conditioning approach are presented.

I. INTRODUCTION

In the recent past, dramatic improvements in productivityhave been realized in the high technology sector as well asin the traditional industrial sector. From a point of view ofthe electric power supply to these segments of loads, thisincrease in productivity has led to a concomitant increase inthe number of loads that are sensitive to power quality. Someof the industries that have such large sensitive loads includesemiconductor manufacturing, textile mills, paper mills andplastic injection molding, etc. Of course, a number of smaller,but equally critical loads such as computers and electronic dataprocessing equipment are also sensitive to power quality. Thetolerance levels of computer equipment are specified by theITI/CBEMA (Information Technology Industry/Computer &Business Equipment Manufacturers’ Association) curves [1].Similar performance specifications for voltage quality havebeen developed for semiconductor manufacturing industriesas well. Fig. 1 illustrates such voltage sag susceptibilitycurves. These curves plot the percent of nominal voltage atthe load terminals as a function of duration in cycles ofthe ac supply (60 Hz). They represent the susceptibility ofsystems to voltage sags that are repetitive and intermittent. TheCBEMA curves represent the boundary of the ac input voltageenvelope that can be tolerated (typically) by most computer-based equipment. When the supply voltage is between theupper curve and the lower curve, the equipment will continueto function normally. When the supply voltage falls outsidethis boundary, the equipment typically stops functioning.

As seen in Fig. 1, the steady state range of tolerance forsensitive equipment is 10% from the nominal voltage, i.e.,the equipment continues to operate normally when sourced byany voltages in this range for an indefinite period. Similarly,

Fig. 1. Curves that indicate acceptable power quality power levels of sensitiveequipment.

voltage swells to magnitude 120% of the nominal value can betolerated for about 0.5 s or 30 cycles; voltage sags to 80% ofnominal for 10 s, i.e., 600 cycles can be tolerated [2]. When thesupply voltage is outside the boundaries of the susceptibilitycurves, improvement of quality of power supplied to sensitiveloads is essential in order to avoid a possible interruption intheir operation. The most straightforward way of ensuringadequate power quality levels is the use of uninterruptiblepower supply (UPS) systems. However, as the size of plantincreases, this solution becomes uneconomical. Alternativesolutions to the problem stem from a careful study of thenature of power quality problems in the field.

Various field power quality surveys indicate that the ma-jority of events that result in process disruption are causedby voltage sags. Moreover, over 90% of voltage disturbancesin the utility lines are single-phase voltage sags caused bymomentary single-line to ground (SLG) faults in distributionsystems [3]. The short circuit faults can be caused by light-ning strokes, storms, animals and other unpredictable factors.Therefore, improvement of power quality levels should beginwith reduction of voltage sags, particularly in a per phasebasis. Although the voltage sag levels can be reduced atthe power system level by oversizing the components of the

0-7803-8269-2/04/$17.00 (C) 2004 IEEE 1445

distribution system installations, often such solutions are toocostly to implement. Alternatively, utilities and customers turnto the practical solution of feeding premium power to sensitiveloads, which is realized by the mitigation equipment at thedistribution feeder close to the customer. This solution is alsocalled Custom Power (CP) [4].

Among CP technologies, both series and shunt deviceshave been applied to solve the voltage sag/dip and flickerproblems and some of these devices are commercialized [5],[6], [7]. Due to the project-oriented nature of CP design, fewgeneral rules can be found in literature. Although extensiveresearch work on the device, either series or shunt, has beencarried out [8], [9], most of them are focused on the device,not at the system level. In this paper, the series and shuntcompensation schemes are studied at the system level. Thepurpose of this system-level study is to reveal more insightsabout these two compensation strategies so that designers ofthe CP device would have some guidance when selectingcompensating strategies at the very beginning of a project.Another motivation of this work is to lay a ground for theapplication of H-bridge multilevel convert to dynamic voltagerestorer (DVR). Different operation strategies are evaluatedand compared regarding the rating issues, operation and faultmodes.

At the outset, one of the first steps in evaluating a particularpower conditioning approach is to determine the VA rating ofthe power converter illustrated as a voltage source converter(VSC) in Fig. 2. Rating of the power converter dependson the load to be serviced, the depth of voltage sag to becompensated and the properties of the power system at the loadlocation. The dependencies among these quantities for the twocompensation approaches are developed in Section II. Oncethe power conditioning system ratings have been determined,their operation during sags can be steered such that there isno active power supplied by the power conditioner, when theoperating conditions permit. These operating boundaries aredetermined and presented in Section III. Furthermore, whenthe operating conditions demand that active power need tobe supplied by the power conditioner, it would be desirableto draw the least possible amount of active power fromthe storage device to maximize the ride-through capability.Operation under these conditions is presented in Section IV.Computer simulation results under this mode of operationare presented in Section V. Experiment with 7-level H-bridgecascaded multilevel converter has been carried out and experi-mental results are included in Section VI. Transitions betweenvarious operating modes, faulted and bypassed conditions arediscussed in the concluding section.

II. POWER CONVERTER RATING - MINIMUM APPARENT

POWER

The simplified single-line equivalent circuits for the seriesand shunt compensation approaches are illustrated in Fig. 3(a)and Fig. 4(a), respectively. The source is modelled by itsThevenin equivalent voltage source Vs and series reactanceXs seen at the point of common coupling (PCC), and the

Fig. 2. Schematic representation of commonly used mitigation approachesto improve voltage sag immunity of sensitive loads: (a) series type; (b) shunttype

load as an impedance ZL, with a power factor angle φ. Forthe purpose of sizing the VA rating of the power conditioner,the series device is modelled as a voltage source Vinj and theshunt device is modelled as a current source Iinj .

For a general load with a wide variation in power factorto be fed under deep sag conditions, the power conditionertypically has to supply active power and reactive power. Thesize of the power conditioner, including interface transformersif any, is determined by the apparent power injection. Withoutany loss of generality, the following assumptions have beenmade to size the power conditioner used as the compensationdevice:

• The magnitude of the load current is 1 per unit. Its phaseangle is referenced to the load voltage.

• The magnitude of the compensated load voltage is main-tained at 1 per unit.

With these assumptions, the problem of minimizing apparentpower turns to minimizing the injected voltage for seriescompensation while minimizing the injected current for shuntcompensation.

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Fig. 3. Illustration of series compensation approach to improving powerquality: (a) single-line equivalent circuit representing key parameters; (b)phasor diagram of salient quantities

A. Series Compensation Strategy

For a series compensation approach, the following phasorequations are readily written from circuit representation asshown in Fig. 3(a).

V s = V pcc + jILXs

V L = V pcc + V inj (1)

V L = IL|ZL|ejφ

where the under-bars (“ ”) with voltage and current variablesindicate they are phasor quantities. In the following text,current and voltage variables without under-bars representcorresponding magnitude of those phasors. The phasor dia-gram illustrating the relationship expressed by (1) is shown inFig. 3(b).

In the phasor diagram, the load current is convenientlychosen as the reference phasor for the series circuit and the linevoltage is represented by the phasor OA, leading the currentby a phase angle φ. It is clear from the phasor diagram, thatthe locus of the voltage at PCC is an arc (with radius = Vs)centered at O′, which is vertically displaced from the centerof the locus arc of vector Vs by ILXs. In this case, the phasorof the injected voltage is collinear with the segment O′A,and represents its minimum value. Other values of injectedvoltage values may be possible while maintaining 1 p.u. outputvoltage, but will not represent a minimum value of injectedvoltage. Using geometric relationships, the magnitude of theinjected voltage under this condition can be expressed as

Vinjmin =√

V 2L + (ILXs)2 + 2VLILXs sin φ − Vs. (2)

Fig. 4. Illustration of shunt compensation approach to improving powerquality: (a) single-line equivalent circuit representing key parameters; (b)phasor diagram of salient quantities

It may be noted that the magnitude of the minimum injectedvoltage increases as the Thevenin impedance of the systemincreases.

B. Shunt Compensation Strategy

In a similar manner, for the shunt compensation approach,the following phasor relations will hold from Fig. 4(a).

V s = V pcc + jILXs

IL = Is + Iinj (3)

V L = IL|ZL|ejφ

The phasor diagram illustrating the above relations is shownin Fig. 4(b).

In the phasor diagram, the load voltage is convenientlychosen as the reference phasor for the parallel circuit andthe load current is represented by the phasor OA, laggingthe voltage by a phase angle φ. The locus of the sourcecurrent is the arc (radius = Vs/Xs) centered at O′, which isvertically displaced from the center of the locus arc of vectorVs by VL/Xs. The minimum injected current lines up withthe segment O′A. In this case, the injected current can beexpressed as

Iinjmin =√

(VL

Xs)2 + I2

L +2VLIL

Xssin φ − Vs

Xs. (4)

It may be noted that the magnitude of the minimum injectedcurrent decreases as the Thevenin impedance of the systemincreases.

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0.5 1 1.5

30

60

90

0

Series Injection with Vs=0.8

VA rating (pu)

φ

(a)

0.5 1 1.5 2 2.5 3

30

60

90

0

Shunt Injection with Vs=0.8

VA rating (pu)

φ

SCR= 8SCR= 4SCR= 2SCR= 1

(b)

Fig. 5. Power converter VA ratings for 20% sag for (a) series (b) shuntcompensation approaches. (Radius corresponds to the VA rating of theconverter and the angle of corresponds to the power factor angle of the loadrespectively.)

C. Comparative Ratings

Based upon the above relationships (2) and (4), the ratingsof the power conditioner may be illustrated in the form of polarplots as shown in Fig. 5, with radius and angle coordinates.In these plots, the radius represents the VA rating of thepower conditioner and the angle represents the lagging (as iscommonly the case in an industrial power system) power factorangle of the load current while correcting a sag of 20% depthbelow the nominal voltage. As may be observed from (2) and(4), the VA ratings depend on the system impedance and hencethe short-circuit ratio (SCR) at PCC. This behavior is readilyevident in the various curves plotted in Fig. 5, for differentvalues of SCR ranging from 1 to 8. It may be observed fromthe figure that the VA rating of the shunt device is greater thanthe series counterpart for the case of SCR larger than unity. Inpractical distribution system, the SCR is generally greater thanthe cases shown in the plots. So the converter sizing favorsseries compensation approach if the voltage correction is themain goal of the application. Therefore, further discussionsare primarily focused on the series compensation strategy,although aspects of the shunt compensation approach will bepointed when appropriate. In the case of a series compensationapproach for a given system (with fixed SCR), the VA ratingdepends on the load power factor and voltage sag depth. Therelation is illustrated in Fig. 6 for series injection strategy withthe power rating represented by the surface.

III. ZERO ACTIVE POWER OPERATION STRATEGY

Once power converter ratings have been fixed on the basisof operating power factor and desired depth of sag to becorrected, various operating modes may be used to mitigatetypical voltage sag conditions. If the load is not sensitive to thevoltage phase jump, it is possible to correct voltage sag withpurely reactive power compensation (RPC). Although RPC

Fig. 6. Variation of power converter VA ratings as a function of power factorand (sagged) source voltage with SCR=8 for a series injection strategy.

Fig. 7. Phasor diagram of salient quantities for series compensation underRPC mode of operation.

mode of operation is limited in terms of voltage sag depth,it does not deplete the power conditioner energy storage, andhence can provide ride through for an infinite period of time.To be sure, the injected values of voltages will have to bewithin the design limits of the converter chosen on the basis ofthe discussion in the previous section. The phasor diagram ofvarious quantities of the system under RPC mode of operationare illustrated in Fig. 7. In this case the injected voltage leadsthe load current by 90 degrees. In contrast, the injected currentlags the load terminal voltage by 90 degrees for the shuntcompensation approach.

The limiting case for series compensation to operate underRPC mode may be determined to be

Vs(min RPC) = VL cosφ. (5)

Furthermore, at the operating condition, the required voltage

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0.5 1 1.5 2

30

60

90

0

Series Injection with Vs=0.8

VA rating (pu)

φ

(P)

SCR=1SCR=2SCR=4SCR=8

Fig. 8. Power converter VA injection for 20% sag under RPC mode ofoperation. (Radius corresponds to the VA rating of the converter and theangle of corresponds to the power factor angle of the load respectively.)

can be found to be

Vinj = VL sin φ −√

V 2s − (VL cosφ)2 + ILXs. (6)

The converter VA injection of series compensation is plottedin Fig. 8 for 20% voltage sag. The polar plots in the figureshow the variation of converter VA ratings for different SCRs,as the load power factor angle varies. It is clear that theoperation region under zero active power compensation modewill be limited by the load power factor. Of course, it will bealso limited by available device ratings.

IV. MINIMUM ACTIVE POWER OPERATION STRATEGY

When the operation conditions are beyond the limits of purereactive power compensation, a minimum active power (MAP)injection strategy may be used so that the energy storageelements can be minimized, or the depletion rate of storedenergy is minimal for servicing the load at a given operatingcondition. In particular, the source voltage falls below thelimits given by (6), the RPC strategy fails, and one needsto resort to active power injection.

To minimize the active power injection from the powerconditioner, active power extraction from the source Vs shouldbe maximized. The phasor diagram of the salient quantities ofthe system under this mode operation is shown in Fig. 9. Forseries compensation, the active power supplied by the sourceis maximized under unity power factor operation, i.e. sourcevoltage is in phase with load current. This minimizes the powerto be supplied from the VSC. This is also clear from the phasordiagram that Vinj has minimum projection on IL. In this case,the injected voltage becomes

Vinj =√

(VL cosφ − Vs)2 + (VL sin φ + ILXs)2. (7)

The converter VA injection required for 20% sag is plottedin Fig. 10. The MAP strategy can be considered to be anextension of RPC. When the converter operating conditionsfall outside the operating region possible with RPC or MAP,

Fig. 9. Phasor diagram of salient quantities for series compensation underminimum active power mode of operation.

0.5 1 1.5 2

30

60

90

0

Series Injection with Vs=0.8

VA rating (pu)

φ

SCR=1SCR=2SCR=4SCR=8

Fig. 10. Power converter VA injection with minimum active power operationstrategy for 20% voltage sag. (Radius and angle of the plot correspond to VArating of the converter and power factor angle of the load, respectively.)

the converter is operated in the minimum apparent power (orbaseline) operating mode.

V. COMPUTER SIMULATION RESULTS

In order to verify the operation of the system accordingto the analytical models, a computer simulation has beencarried out for a series power conditioning device usingMatlab and Simulink [10], [11]. The block diagram of thecontroller is shown in Fig. 11. Selected results from the systemoperating under RPC mode are shown here. The structure ofthe controller and its design are beyond the scope of the currentpaper and are discussed elsewhere [12].

The parameters of the system used in the simulation are:Ls = 1.4 mH, L = 10 mH, R = 2 Ω, VAC = 480/

√3

V(rms). These parameter settings correspond to SCR = 8.0,load power factor= 0.47. The simulation results with a 20%voltage sag in three phases are illustrated in Fig. 12. The figureillustrates the source voltage, injected voltage, load voltage,load current and inverter dc bus voltage. After transient, only

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Fig. 11. Controller block diagram

reactive power is injected, which can be seen from the steadystate settlement of dc bus voltage, which is purely capacitive.The injection voltage is about 103 V(rms), or 0.37 pu. Thisresult is indicated as point ’P’ in Fig. 8. It is clear this resultmatches the prediction. Although the analytical model and thecontroller have been developed only for a balanced three phasesystem, the performance of the system for a more typicalsystem with unbalanced sag may be studied readily usingcomputer simulations. The response of the system With thesame controller under a single phase voltage sag is shown inFig. 13.

VI. EXPERIMENTAL RESULTS

A laboratory prototype voltage conditioning system hasbeen developed using a H-bridge cascaded multilevel con-verter. The converter has been used to verify the predictionsfrom the analytical modelling during operation in RPC mode.The parameters of the laboratory prototype system (in pu),with a base power of 35 VA and a base voltage of 25 V, arelisted in Table I.

TABLE I

PARAMETERS USED IN EXPERIMENT.

Source impedance Xs 0.115

Load impedance ZL 1.0

Load power factor 0.5

Selected traces from the experimental system are shownin Fig. 14. The injected voltage (the 2nd trace) leads theload current (the 4th trace) by 90 degrees, indicating thatonly reactive power is being injected from the VSC. Thesource voltage Vs = 0.115 with the load voltage and loadcurrent both are one per unit. With the measured quantities andparameters given in Table I, the series injection voltage can be

0 5 10 15−500

0

500

Sou

rce

volta

ge

0 5 10 15−200

0

200

Inje

ctio

n vo

ltage

0 5 10 15−500

0

500

Load

vol

tage

0 5 10 15−100

0

100

Load

cur

rent

0 5 10 15660

680

700

720

DC

bus

t

abc

abc

abc

abc

data 1

Fig. 12. Computer simulation results of the series compensation approachunder a three-phase balanced sag (the plots are source voltages, series injectionvoltages, load terminal voltages, load currents and device internal dc busvoltage, respectively, from top to bottom.)

0 5 10 15−500

0

500

Sou

rce

volta

ge

0 5 10 15−200

0

200

Inje

ctio

n vo

ltage

0 5 10 15−500

0

500

Load

vol

tage

0 5 10 15−100

0

100

Load

cur

rent

0 5 10 15680

690

700

710

DC

bus

t

abc

abc

abc

abc

vdc

Fig. 13. Computer simulation results of the series compensation approachunder a single-phase voltage sag (the plots are in the same order as in Fig. 12).

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Fig. 14. Traces of source voltage, series injected voltage, load terminalvoltage and load current, respectively, from top to bottom obtained from thelaboratory prototype system.

calculated as 0.741 pu by use of equation (6). The measuredinjection voltage is 0.752 pu. This small error between theanalytical predictions and experimental measurements (about1.5%) may be attributed to parameter uncertainties and lossesin the system.

VII. CONCLUSIONS

The application power quality conditioning devices to meetstringent demands of sensitive loads is continuing to grow.Although series and shunt compensation approaches for pro-viding power quality conditioning have been proposed anddemonstrated, clear association between operating scenariosand their characteristics have not been definitively known. Thispaper has presented a simple but useful analytical modellingapproach to quantify the the power converter ratings requiredfor the two approaches. On the basis of the modelling results,it may be concluded that series power conditioning approachis suitable for strong ac systems and shunt power conditioningapproach is suitable for weak ac systems.

Based on the analytical model different operating strategiesthat either eliminate the need for real power exchange fromthe power converter, minimize the amount of real powerexchange, or minimize the apparent power exchange have beenidentified, depending on the sag level and load power factor. Adetailed computer simulation model for the system includingthe dynamic and control elements has been developed and usedto verify the analytical model. A laboratory scale experimentalprototype system has also been used to confirm the resultsfrom the computer simulations.

Other operating issues such as storage system management,fault management, bypass, mode transitions, controller designissues etc. are current issues being studied further and theresults from these studies will be presented in future publica-tions.

ACKNOWLEDGMENT

The authors would like to acknowledge support from Wis-consin Electric Machine and Power Electronics Consortium(WEMPEC) at the University of Wisconsin-Madison. Thework made use of ERC shared facilities supported by theNational Science Foundation (NSF) under AWARD EEC-9731677. The laboratory prototype system used for the exper-imental work was developed by Mr. Yusuke Fukuta throughsupport from the Office of Naval Research through grantN00014-01-1-0623.

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[10] Using Matlab, The MathWorks Inc, Natick, MA, 2003.[11] Using Simulink, The MathWorks Inc, Natick, MA, 2003.[12] G. Venkataramanan and B. Wang, “Dynamic modeling and control of

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